QuickBooks already has a company file open. If you want to open another company file, close this one first. Only QuickBooks Accountant and QuickBooks Enterprise Solutions can open two company files at once.
This occurs if either QuickBooks or Atlas is set to 'Run As Administrator' while the other program is not. You can easily check/change this setting by right-clicking on either applications icon on the desktop and selecting 'Properties'. On the 'Compatibility' tab you can check the box for 'Run this Program as an Administrator'.